This invention relates generally to manufacturing processes for semiconductor devices having GaAs/Ge heteroface junctions, and more particularly in one embodiment to fabrication of multi-junction solar cells.
Many types of heteroface junctions have been fabricated. Generally these junctions have been used for ultra high speed switching, microwave amplification, and binary memory. III-V/Ge heterofaces, in particular, have been investigated including theoretical analysis and fabrication approaches. Much information can be found in the literature on the subject. An excellent compendium of information can be obtained from the text book "Physics of Semiconductor Devices" by Simon M. Sze, Wiley-Interscience.
Some heteroface junctions have been considered for cascaded dual bandgap solar cells. The GaAs on Ge heterojunction has been investigated to some extent.
Efforts in solar cell technology include, among other things, attempts to increase the solar cell conversion efficiency, and one approach to this problem has been the utilization of stacked or tandem connected semiconductor wafers in a structure responsive to different wavelength ranges of sunlight. The stacked PN junctions will generate a combined power which is greater than the individual power outputs from the individual PN junctions.
A second approach to the problem of increasing the efficiency and output power of solar cells is to epitaxially grow multiple layers of different III-V semiconductor compound materials vertically upward from one surface of a suitable substrate material, see for example U.S. Pat. No. 4,017,332 by James. U.S. Pat. No. 4,128,733 by Fraas et al covers a single-wafer dual-band-gap solar cell including a gallium arsenide/germanium heteroface structure.
U.S. Pat. No. 3,271,637 discloses, inter alia, fabrication of a GaAs solar detector comprising vapor-depositing a film of N-type gallium arsenide directly upon a manganese substrate whereby a slight amount of manganese diffusion into the gallium arsenide is effected at the gallium arsenide-manganese interface resulting in a P-N junction within the deposited gallium arsenide film. The James U.S. Pat. No. 4,017,332 discloses that during epitaxial growth of one layer (InP P-doped with zinc or magnesium) on another layer (InGaAs), and during growth of subsequent layers, some of the P-type dopant in the first layer will diffuse into the other layer due to the high temperatures used in growth thereby forming an opposite conductivity junction level within the other layer and just below its junction with the first layer. Similarly, other layers and junctions are produced.
U.S. Pat. No. 4,126,930 by Moon relates to semiconductive materials useful in devices such as solar voltaic cells and involves magnesium diffusion during epitaxial growth of a magnesium doped AlGaAs layer of an underlying GaAs layer producing a thin layer of Mg-doped P-type GaAs and a P-N junction with the underlying N-type GaAs.
Other art of general interest includes my U.S. Pat. Nos. 4,070,205 and 4,116,717, U.S. Pat. No. 4,179,702 by Lamorte, and U.S. Pat. No. 3,368,125 by Pasierb.